The invention relates to a method of manufacturing a tube of transparent, translucent or opaque fused silica to close tolerances as regards its outside diameter, in which method a plastic, tubular section of the horizontally disposed fused silica tube produced by drawing a raw material, in the form of a hollow cylinder for example, in a drawing zone and rotating it about its longitudinal axis, is blown against a graphite plate which determines the outside diameter of the tube; it relates also to an apparatus for the practice of this method.
German Pat. No. 742,008 describes a method of making, from glass tubes, glass bodies having a precisely specified diameter which varies from section to section according to specifications. The starting point is a glass tube of given length. This glass tube is heated along the section that is to be shaped, until it softens, and then it is drawn through a gauge corresponding to the diameter that is to be produced. The entire section that is to be drawn can be heated as the drawing is performed, the drawing being accomplished with the use of a burner acting on one side while the glass tube is rotated, it being desirable to rotate the gauge at the same angular speed. While the softened glass tube is being drawn through the gauge, the tube is blown against the gauge, thereby producing a precise application of the tube to the gauge. To solidify the glass tube immediately after passage through the gauge, either a cooling jet is provided, or the gauge itself is provided with cooling passages to carry a coolant.
The German Patent application B No. 17,145, published on July 9, 1953, discloses a method of making moldings of comparatively large cross section from fused silica. In this case, crystalline silica is continuously fed through a passage having a melting zone, and is fused to form a strand having the same cross-sectional area as the profile that is to be produced. After leaving the passage, the strand is severed into appropriate segments. The raw material and the melt can be transported through a horizontally disposed passage. By this method, any desired solid or hollow profile can be made. In the final section of the passage, as long as the material is still sufficiently plastic, it can be shaped by an action similar to that of the die of an extruding machine. The strand leaving the passage, if it is still in the plastic state, can also be subjected to additional forming, for example by rolling, pressing or the like.
Lastly, it is also known to calibrate fused silica tubes by mounting cut lengths of tubing in a redrawing machine, heating them zone-wise until they become plastic, and, while rotating them about their longitudinal axis, blowing them against a graphite plate whose distance from the longitudinal axis of the tube corresponds to one-half of the specified outside diameter.